Make tests use ReactTestUtils.renderIntoDocument

@zpao - this PR only has about a 1/3 of the tests converted where it seemed applicable.  I wanted to start a discussion about a situation I ran into multiple times and wasn't sure what the desired way was to go about.

In some tests such as in ReactCSSTransitionGroup (and many others) there's a need to perform operations on the original container that the component got mounted on.  So far I've used something like
```
var instance = ReactTestUtils.renderIntoDocument(<Component />);
var container = ReactMount.findReactContainerForID(instance._rootNodeID);
```

@syranide points out and rightfully so that access to private instance's ID will probably not be available in ES6 classes which brings the question of whether or not we should make a helper method for this use case or come up with a better alternative.  

I'm open to feedback on this and I'm referencing issue #1250